Nickel-mediated N–N bond formation and N2O liberation via nitrogen oxyanion reduction

The syntheses of (DIM)Ni(NO₃)₂ and (DIM)Ni(NO₂)₂, where DIM is a 1,4-diazadiene bidentate donor, are reported to enable testing of bis boryl reduced N-heterocycles for their ability to carry out stepwise deoxygenation of coordinated nitrate and nitrite, forming O(Bpin)₂. Single deoxygenation of (DIM)Ni(NO₂)₂ yields the tetrahedral complex (DIM)Ni(NO)(ONO), with a linear nitrosyl and κ¹-ONO. Further deoxygenation of (DIM)Ni(NO)(ONO) results in the formation of dimeric [(DIM)Ni(NO)]₂, where the dimer is linked through a Ni–Ni bond. The lost reduced nitrogen byproduct is shown to be N₂O, indicating N–N bond formation in the course of the reaction. Isotopic labelling studies establish that the N–N bond of N₂O is formed in a bimetallic Ni₂ intermediate and that the two nitrogen atoms of (DIM)Ni(NO)(ONO) become symmetry equivalent prior to N–N bond formation. The [(DIM)Ni(NO)]₂ dimer is susceptible to oxidation by AgX (X = NO₃⁻, NO₂⁻, and OTf⁻) as well as nitric oxide, the latter of which undergoes nitric oxide disproportionation to yield N₂O and (DIM)Ni(NO)(ONO). We show that the first step in the deoxygenation of (DIM)Ni(NO)(ONO) to liberate N₂O is outer sphere electron transfer, providing insight into the organic reductants employed for deoxygenation. Lastly, we show that at elevated temperatures, deoxygenation is accompanied by loss of DIM to form either pyrazine or bipyridine bridged polymers, with retention of a BpinO⁻ bridging ligand.

Deoxygenation of nitrogen oxyanions coordinated to nickel using reduced borylated heterocycles leads to N–N bond formation and N₂O liberation. The nickel dimer product facilitates NO disproportionation, leading to a synthetic cycle.     

Radical generation enabled by photoinduced N–O bond fragmentation

Recent advances in synthetic chemistry have seen a resurgence in the development of methods for visible light-mediated radical generation. Herein, we report the development of a photoactive ester based on a quinoline N-oxide core structure, that provides a strong oxidant in its excited state. The heteroaromatic N-oxide provides access to primary, secondary, and tertiary radical intermediates, and its application toward the development of a photochemical Minisci alkylation is reported.

Recent advances in synthetic chemistry have seen a resurgence in the development of methods for visible light-mediated radical generation.

Photoinduced radical generation has become a focal point of contemporary chemical research. Historically, the photochemical formation of radicals has been achieved via the direct irradiation of organic chromophores with high energy UV light, with notable examples including Norrish – type 1 reactions and the photochemical decomposition of azo compounds, peroxides, N-(acyloxy)-pyridones, and xanthates.¹ While a powerful tool, the propensity of organic functional groups to absorb UV light leads to undesired excitation events that ultimately result in uncontrolled, deleterious reactivity.² The use of visible light irradiation to drive reactivity offers a solution to the problems presented by UV light photochemistry, as typical organic functionalities do not absorb in the visible region of the electromagnetic spectrum. In this regard, photoredox catalysis has emerged as a powerful tool for the generation of radical intermediates, as the visible light absorbing catalysts can access high energy excited states that engage organic substrates in redox events.³ While incredibly versatile, the manipulation of substrate oxidation states can limit the scope of reactivity, as functionalities that are predisposed to oxidation (or reduction) may undergo undesired side reactions. To circumvent this limitation, redox auxiliaries are employed to alter the redox properties of the substrate and facilitate reactivity under mild reducing (or oxidizing) conditions.⁴Radical formation via photoinduced dissociation of an auxiliary, or complex, represents a complementary strategy that, in principle, should be tolerant of redox sensitive functionalities. However, despite the ubiquity of this strategy in UV light mediated reaction manifolds, the development of reagents that undergo efficient photodissociation upon visible light irradiation is limited. In 2017, Melchiorre and coworkers reported the dissociation of 4-alkyl-1,4-dihydropyridines (alkyl-DHPs) upon irradiation with 405 nm light (Fig. 1A).⁵ Study of the photoactive alkyl-DHPs revealed that they are reductants in their excited state (D⁺˙/D* = −2.0 V vs. SCE) and, as such, both the dissociative and redox properties of the excited state could be exploited for ipso-substitution of aryl nitriles and nickel catalyzed C(sp²)–C(sp³) cross-coupling reactions.Open in a separate windowFig. 1Overview of photochemical radical generation in synthetic chemistry.In 2020, Ohmiya and colleagues reported that boracene, when reacted with organolithium or Grignard reagents, forms a photoactive alkyl borate salt that liberates an equivalent of an alkyl radical upon irradiation with 440 nm light (Fig. 1B).⁶ Characterization of the boracene-based alkyl borate revealed a strongly reducing excited state (B⁺˙/B* = −2.2 V vs. SCE), and this auxiliary was demonstrated to be effective for nickel catalyzed C(sp²)–C(sp³) cross-coupling reactions with tertiary alkyl radicals.Pioneering work by Barton and coworkers in the 1980''s demonstrated that the thermal or photochemical decomposition of N-(acyloxy)-2-thiopyridones (commonly referred to as Barton esters) could efficiently generate carbon centered radicals, ultimately delivering the corresponding alkane or thioether products.⁷ However, state-of-the-art methods employing pyridine N-oxide or its derivatives as radical precursors have demonstrated limited intermolecular reactivity, as the generated radical intermediates are competitively trapped by the pyridine-based auxiliary.^8,9 To overcome these limitations, we sought to design a pyridine N-oxide based auxiliary that does not undergo undesired alkylation reactions. Inspired by Barton''s work and informed by our previous studies,^8,9 we envisioned that the fast fragmentation of N–O bonds in pyridine N-oxide derivatives could be leveraged in the design of a photocleavable activator that would deliver carbon centered radicals from readily available carboxylic acids as precursors. Importantly, the development of a photoactive ester derived from simple pyridine N-oxide core structure delivers a species that is a strong oxidant in its excited state, complementing the reductive excited states of the previously developed photoactive auxiliaries. Moreover, by using a core structure that is derived from an abundant heteroaromatic building block, the designed photoactive esters will contain highly tunable core structures, allowing for control over the photoexcitation and fragmentation events. Herein, we report the development of a photoactive ester derived from a quinoline N-oxide core structure and its application to achieve an efficient intermolecular Minisci alkylation (Fig. 1C).At the outset of our study, we established several requirements to be satisfied by potential photoactive esters (Fig. 2B): (1) the pyridine N-oxide core structure needed to be preserved, (2) the heteroaromatic core would need to have blocking substituents at the sites of alkylation to slow deleterious functionalization of the ester (i.e. 2-, 4-, and 6-substituted pyridine N-oxide derivatives), (3) the heteroaromatic N-oxide would need to maintain sufficient nucleophilicity to form the activated N-acyloxy pyridinium, (4) the heteroaromatic backbone would need to deliver a sterically accessible N-oxide functionality. Additionally, it was recognized that pyridine N-oxide derivatives bearing alkyl substituents with benzylic C–H bonds were not suitable photoactive esters, as a deleterious Boekelheide reaction occurred upon acylation of the N-oxide functionality. Ultimately, 2,4,6-triphenyl pyridine N-oxide (TPPNO), methyl 2-phenylquinoline-4-carboxylate N-oxide (PQCNO), and methyl acridine-9-carboxylate N-oxide (ACNO) were identified as potential photoactive ester precursors that met all aforementioned requirements.Open in a separate windowFig. 2(A) Comparison of pyridinium esters. (B) Overview of photoactive ester design principles. (C) Photophysical characterization of Ac-TPPNO, Ac-PQCNO, and Ac-ACNO.Initial investigations focused on the photophysical characterization of the three heteroaromatic N-oxides (Fig. 2C). Uv-vis spectroscopy in acetonitrile revealed TPPNO to have two maximum absorbances at 319 nm and 365 nm, with the latter absorbance tailing off beyond 400 nm. PQCNO and ACNO also displayed a strong absorbance feature at 365 nm, however, for ACNO two additional lower energy absorbance features at 418 nm and 444 nm were observed. Addition of acetyl chloride to TPPNO and PQCNO resulted in an increase of intensity for the absorbance features, with no apparent shift in the absorbance maxima. Acylation of the ACNO auxiliary with acetyl chloride caused the maximum at 274 nm to diverge into two distinct sharp absorbance features with maxima at 264 nm and 275 nm. Additionally, the absorbance feature at 364 nm both increased in intensity and became more structured upon acylation, revealing an apparent shoulder at 352 nm. Measurement of the fluorescence spectra for the heteroaromatic N-oxides (irradiated at 361 nm) revealed that Ac-TPPNO, Ac-PQCNO, and Ac-ACNO have emission maxima at 435 nm, 468 nm and 519 nm, respectively (Fig. 3A). In the absence of an acyl equivalent, no emission was observed for the heteroaromatic N-oxides. This finding is consistent with previous reports that pyridine N-oxides and quinoline N-oxides possess non-emissive excited states under basic conditions due to a propensity to undergo a fast rearrangement on the singlet surface.¹⁰Open in a separate windowFig. 3(A) Absorbance and emission spectra for Ac-TPPNO, Ac-PQCNO, and Ac-ACNO. Emission recorded upon irradiation at 361 nm. (B) Fluorescence spectra of Ac-PQCNO monitored over 25 successive scans (excitation 335 nm). (C) Decomposition of Ac-PQCNO upon irradiation by a 427 nm Kessil lamp, monitored by Uv-vis. (D) Proposed mechanism for photochemical decomposition of Ac-PQCNO.Investigation of the photoinduced N–O bond cleavage of Ac-PQCNO revealed that upon successive fluorescence scans (excitation at 335 nm), Ac-PQCNO is observed to decompose steadily with the concomitant appearance of the fluorescence signal corresponding to the generation of deoxygenated quinoline (Fig. 3B). Interrogation of the decomposition by Uv-vis spectroscopy revealed that after 60 s of irradiation of Ac-PQCNO with a 427 nm Kessil lamp, a decrease in the absorbance feature at 365 nm was observed. Upon extending irradiation time out to 30 min, significant degradation of the Ac-PQCNO was observed.Together, the photophysical characterization shows the photoactive esters possess conserved absorption features at 365 nm. Consistent with previous studies on quinoline N-oxide photochemistry, the absorption feature at 365 nm is thought to be π,π* in character and responsible for the observed deoxygenation reactivity.¹¹ The deoxygenation of the aromatic N-oxides is hypothesized to arise from crossing over of the π,π* excited state to a dissociative π,σ* state. Computational evidence in support of the hypothesized mechanism can be found in Fukuda and Ehara''s study on the deoxygenation of structurally related N-hydroxypyridine-2(1H)-thione.¹² Experimental support for this proposed mechanism can be found in Hata and Tanaka''s study of the gas phase photolysis of pyridine N-oxide¹³ as well as Hata''s subsequent translation of this work to the solution phase photodeoxygenation of heteroaromatic N-oxides in the presence of a strong Lewis acid, BF₃·Et₂O.¹⁴Interrogation of the thermal reactivity of Ac-PQCNO revealed there was no deoxygenation of the heteroaromatic N-oxide after heating to 90 °C for 3 hours. This demonstrates that the remarkable reactivity of Ac-PQCNO is due to dissociation of the N–O bond from a low energy photoexcited state.Electrochemical analysis of the heteroaromatic N-oxide photoactive esters using cyclic voltammetry in acetonitrile revealed the Ac-TPPNO, Ac-PQCNO, and Ac-ACNO to have low energy reduction waves, with measured E_1/2 of −0.79 V vs. SCE, − 0.45 V vs. SCE, and −0.47 V vs. SCE. Applying information gathered from absorbance and emission spectroscopy, as well as the electrochemical data, the standard potential for oxidation of Ac-TPPNO, Ac-PQCNO and Ac-ACNO in the excited state was estimated to be +2.30 V [E⁰(T⁺*/T˙)], +2.57 V [E⁰(P⁺*/P˙)], and +2.32 V [E⁰(A⁺*/A˙)] vs. SCE according to the Rehm–Weller approximation (Fig. 2C).¹⁵Due to the mild photolytic conditions for decarboxylative radical generation from N-oxides and characteristics as excited state oxidants, we sought to assess the reactivity of the photocleavable esters towards the development of an intermolecular Minisci alkylation (Fig. 4). Initial studies focused on the addition of the tert-butyl radical to 4-chloroquinoline in acetonitrile. Assessment of reactivity revealed TPPNO and PQCNO to perform identically under unoptimized conditions delivering 27% of the desired 2-tert-butyl-4-chloroquinoline product, while ACNO produced less than 5% of the desired product. Due to the low cost of the parent quinoline and its high yielding N-oxidation,¹⁶ we elected to focus our investigations on the application of PQCNO as a photoactive ester. Degassing the reaction resulted in a substantial increase in yield, providing 90% isolated yield of the desired product. Further investigation of solvents and additives did not result in increased reactivity.Open in a separate windowFig. 4Scope of intermolecular Minisci alkylation. Isolated yields unless otherwise noted. Standard conditions: substrate (0.2 mmol, 1 equiv.), PQCNO (2 equiv.), acyl chloride (2.2 equiv.), CaCl₂ (1 equiv.), MeCN (0.5 mL), N₂ atmosphere, 427 nm Kessil lamp, 30 min. ^aReaction run for 1 hour. ^bReaction run for 2 hours. ^cReaction run for 1 hour with a 1 : 1 mixture of MeCN to DCM as solvent. NMR yield with methyl tert-butyl ether as internal standard, in brackets.Assessment of the scope for the intermolecular Minisci alkylation revealed the tert-butyl radical addition was the most efficient of the simple alkyl fragments (90%, 4a), followed by isopropyl addition (50%, 4b), ethyl addition (48%, 4c), and finally methyl addition (43%, 4d) (Fig. 4). Simple carbocyclic radical fragments such as cyclohexyl (4e), cyclobutyl (4f), and cyclopropyl (4g) provided the corresponding alkylation products in 88%, 57%, and 34% yield, respectively. Interestingly, Minisci alkylation products from more complex carbocyclic radical fragments were also accessible under-developed reaction conditions as 1-phenylcyclopropyl (4h), adamantyl (4i), 4-(methoxycarbonyl)-[2.2.2]-bicycloctyl (4j), and 3-(methoxycarbonyl)-[1.1.1]-bicyclopentyl (4k) fragments gave the corresponding radical addition products in 37–80% yield. Application of tetrahydropyran-4-carbonyl chloride (4l), N-Boc azetidine 3-carbonyl chloride (4m), N-Boc piperidine 4-carbonyl chloride (4n), and N-Boc piperidine 3-carbonyl chloride (4o) provided the desired coupling products in 41–61% yield, respectively.Assessment of the heterocyclic coupling partners revealed that substituted pyridine and quinoline derivatives performed well in the Minisci alkylation, however, over-alkylation of the heteroarene was often observed (4p–4t). Notably, 4-cyanopyridine (4p) and lepidine (4s) performed well under the developed intermolecular reaction conditions; these substrates were either low yielding or inaccessible under our previously reported fragment coupling conditions.⁸ Modestly complex heteroaromatic scaffolds such as quinoxaline (4u), and 3-chloro-6-phenylpyridazine (4v) also performed well under the tert-butylation reaciton conditions. Biologically active scaffolds such as the 4-chloroquinazoline core of erlotinib (4w) and the imidazopyrazine core structure of gandotinib (4x) each provided a single regioisomer of the tert-butyl addition product in high yield. Finally, nicotine (4y) was observed to undergo tert-butyl addition in 37% yield with retention of the configuration at the benzylic stereocenter.During the exploration of the Minisci alkylation reaction, it was realized that the deoxygenated quinoline could be recovered in high yields (87% recovery). Resubjecting the recovered quinoline to oxidation conditions, PQCNO could be re-generated in 71% yield. Regenerated PQCNO was observed to show no decrease in reactivity when recycled through three consecutive reactions (see ESI^†).To probe the mechanism of the photoinduced Minisci alkylation reaction, we monitored the reaction by employing an in situ LED NMR device equipped with a 430 nm LED source. Investigation of the N–O bond fragmentation revealed that the rate of deoxygenation was not dependent on the rate of decarboxylation for the acyloxy group. Benzoyloxy and pivaloyloxy substituted N-oxides provided similar rates of deoxygenation (k_obs = −5.8 × 10⁻⁵ M s⁻¹ (BzO–) vs. k_obs = −6.3 × 10⁻⁵ M s⁻¹ (PivO–); see ESI^†), a result that is interesting given the difference in rates of decarboxylation form the respective carboxylic acids.^17,18 Alteration of the aryl substituent in the 2-position of the PQCNO auxiliary was observed to impact the rate of deoxygenation, as ortho substituted 2,6-dimethylphenyl substituent provided an increased rate of deoxygenation (k_obs = −7.9 × 10⁻⁵ M s⁻¹), whereas electron rich para-methoxyphenyl substituent led to a substantial decrease in the observed rate for deoxygenation (k_obs = −2.2 × 10⁻⁵ M s⁻¹).Monitoring the deoxygenation of Piv-PQCNO under reaction conditions revealed an increase in the observed rate of deoxygenation in the presence of 4-chloroquinoline (Fig. 5A). We hypothesize that the change in rate for deoxygenation is indicative of a propagative reaction mechanism in which electron transfer from the radical addition product (III) to acylated PQCNO (I) leads to formation of the final C–H alkylated product as well as generating a second equivalent of R˙ through a reductive decarboxylation of acylated PQCNO (I). Determination of the quantum yield (Φ) for the decomposition of Piv-PQCNO supported the proposed propagative chain mechanism, in the presence of 4-chloroquinoling Φ = 0.983 whereas in the absence of a substrate Φ = 0.218 for Piv-PQCNO decomposition. The increase in observed quantum yield is indicative of chain mechanism promoting Piv-PQCNO decomposition in the presence of a substrate. Further support was found when subjecting 2-phenylpropionyl chloride to the reaction conditions in the absence of a heteroaromatic substrate, 1-chloro-1-phenylethane was observed (see ESI^†), demonstrating the ability of acyl PQCNO to oxidize stabilized radical intermediates.Open in a separate windowFig. 5(A) Reaction profile in the presence (blue) and absence (yellow) of substrate, monitored using 430 nm LED NMR apparatus. (B) Proposed mechanism for photomediated Minisci reaction.¹⁹On the basis of these findings, we propose the following mechanism (Fig. 5B). Initiation of the Minisci alkylation occurs by the photoinduced decomposition of acyl-PQCNO (I) to generate an equivalent of a reactive radical intermediate (R˙). Addition of R˙ to an equivalent of the protonated heteroaromatic substrate provides intermediate (II). Deprotonation of (II) generates radical intermediate (III) that is in turn oxidized by a second equivalent of acyl-PQCNO (I), providing the desired C–H alkylated product while generating a second equivalent of R˙ through the decomposition of reduced acyl-PQCNO (IV), thereby propagating the reaction.Finally, we sought to explore alternate radical transformations that can be promoted by PQCNO-based esters (Scheme 1). Lactonization of 2-phenylbenzoyl chloride was carried out, providing moderate yield for the 3,4-benzocoumarin product.²⁰ By employing trifluoroacetic anhydride (TFAA) as an acyl equivalent in the presence of tert-butyl anisole, radical trifluoromethylation of the electron-rich arene was achieved in moderate yields.⁸ Further assessments of reactivity are currently ongoing within our laboratory.Open in a separate windowScheme 1Alternate radical transformations.In conclusion, we have developed a photoactive ester based upon the quinoline N-oxide core which delivers a strong oxidant in its excited state. The designed photocleavable ester enabled the development of a photochemical Minisci alkylation, providing a reaction platform that leveraged both the photochemical dissociation and the oxidizing characteristics of the photoactive esters. The photochemical reactivity of the PQCNO ester was also demonstrated to effect radical lactonization and trifluoromethylation reactions.     

Influence of the solvent in the self-assembly and binding properties of [1 + 1] tetra-imine bis-calix[4]pyrrole cages

We report the self-assembly of shape-persistent [1 + 1] tetra-imine cages 1 based on two different tetra-α aryl-extended calix[4]pyrrole scaffolds in chlorinated solvents and in a 9 : 1 CDCl₃ : CD₃CN solvent mixture. We show that the use of a bis-N-oxide 4 (4,4′-dipyridyl-N,N′-dioxide) as template is not mandatory to induce the emergence of the cages but has a positive effect on the reaction yield. We use ¹H NMR spectroscopy to investigate and characterize the binding properties (kinetic and thermodynamic) of the self-assembled tetra-imine cages 1 with pyridine N-oxide derivatives. The cages form kinetically and thermodynamically stable inclusion complexes with the N-oxides. For the bis-N-oxide 4, we observe the exclusive formation of 1 : 1 complexes independently of the solvent used. In contrast, the pyridine-N-oxide 5 (mono-topic guest) produces inclusion complexes displaying solvent dependent stoichiometry. The bis-N-oxide 4 is too short to bridge the gap between the two endohedral polar binding sites of 1 by establishing eight ideal hydrogen bonding interactions. Nevertheless, the bimolecular 4⊂1 complex results as energetically favored compared to the 5₂⊂1 ternary counterpart. The inclusion of the N-oxides, 4 and 5, in the tetra-imine cages 1 is significantly faster in chlorinated solvents (minutes) than in the 9 : 1 CDCl₃ : CD₃CN solvent mixture (hours). We provide an explanation for the similar energy barriers calculated for the formation of the 4⊂1 complex using the two different ternary counterparts 5₂⊂1 and (CD₃CN)₂⊂1 as precursors. We propose a mechanism for the in–out guest exchange processes experienced by the tetra-imine cages 1.

We report the self-assembly and binding properties of [1+1] tetra-imine cages based on two different tetra-α aryl-extended calix[4]pyrrole scaffolds in chlorinated solvents and in a 9 : 1 CDCl₃ : CD₃CN solvent mixture.     

A bis(silylene)pyridine pincer ligand can stabilize mononuclear manganese(0) complexes: facile access to isolable analogues of the elusive d7-Mn(CO)5 radical

Using the potentially tridentate N,N′-bis(N-heterocyclic silylene)pyridine [SiNSi] pincer-type ligand, 2,6-N,N′-diethyl-bis[N,N′-di-tert-butyl(phenylamidinato)silylene] diaminopyridine, led to the first isolable bis(silylene)pyridine-stabilized manganese(0) complex, {κ³-[SiNSi]Mn(dmpe)} 4 (dmpe = (Me₂P)₂C₂H₄), which represents an isolobal 17 VE analogue of the elusive Mn(CO)₅ radical. The compound is accessible through the reductive dehalogenation of the corresponding dihalido (SiNSi)Mn(ii) complexes 1 (Cl) and 2 (Br) with potassium graphite. Exposing 4 towards the stronger π-acceptor ligands CO and 2,6-dimethylphenyl isocyanide afforded the related Mn(0) complexes κ²-[SiNSi]Mn(CO)₃ (5) and κ³-[SiNSi]Mn(CNXylyl)₂(κ¹-dmpe) (6), respectively. Remarkably, the stabilization of Mn(0) in the coordination sphere of the [SiNSi] ligand favors the d⁷ low-spin electronic configuration, as suggested by EPR spectroscopy, SQUID measurements and DFT calculations. The suitability of 4 acting as a superior pre-catalyst in regioselective hydroboration of quinolines has also been demonstrated.

An isolable bis(silylene)pyridine stabilized manganese(0) complex {κ³-[SiNSi]Mn(dmpe)}, isolobal to elusive Mn(CO)₅ radical has been synthesized and fully characterised.     

Sequence-independent activation of photocycloadditions using two colours of light

We exploit two reactive chromophores to establish sequence-independent photochemical activation, employing ortho-methyl benzaldehyde (oMBA) and N,N-(dimethylamino)pyrene aryl tetrazole (APAT) with N-(2-hydroxy)ethyl maleimide (NHEM), without any additives. Critically, the order of the irradiation sequence is irrelevant, as the shorter wavelength does not activate the higher wavelength activated species. Therefore, full sequence-independent λ-orthogonality is achieved through differences in both the reaction quantum yields (Φ_r,oMBA and Φ_r,APAT) and wavelength-dependent reactivity profiles of the employed chromophores.

We exploit two reactive chromophores to establish sequence-independent photochemical activation without any additives.     

Phosphite mediated asymmetric N to C migration for the synthesis of chiral heterocycles from primary amines

A phosphite mediated stereoretentive C–H alkylation of N-alkylpyridinium salts derived from chiral primary amines was achieved. The reaction proceeds through the activation of the N-alkylpyridinium salt substrate with a nucleophilic phosphite catalyst, followed by a base mediated [1,2] aza-Wittig rearrangement and subsequent catalyst dissociation for an overall N to C-2 alkyl migration. The scope and degree of stereoretention were studied, and both experimental and theoretical investigations were performed to support an unprecedented aza-Wittig rearrangement–rearomatization sequence. A catalytic enantioselective version starting with racemic starting material and chiral phosphite catalyst was also established following our understanding of the stereoretentive process. This method provides efficient access to tertiary and quaternary stereogenic centers in pyridine systems, which are prevalent in drugs, bioactive natural products, chiral ligands, and catalysts.

N-Alkylpyridinium salt of chiral amines undergoes phosphite mediated stereoretentive migrations to generate chiral alkylpyridines. The role of phosphite on reactivity and stereoselectivity were examined to achieve a catalytic asymmetric version.     

Iterative addition of carbon nucleophiles to N,N-dialkyl carboxamides for synthesis of α-tertiary amines

A protocol for the synthesis of α-tertiary amines was developed by iterative addition of carbon nucleophiles to N,N-dialkyl carboxamides. Nucleophilic 1,2-addition of organolithium reagents to carboxamides forms anionic tetrahedral carbinolamine (hemiaminal) intermediates, which are subsequently treated with bromotrimethylsilane (Me₃SiBr) followed by organomagnesium (Grignard) reagents, organolithium reagents or tetrabutylammonium cyanide, affording α-tertiary amines. Employment of (trimethylsilyl)methylmagnesium bromide as the 2^nd nucleophile allowed for aza-Peterson olefination of the resulting α-tertiary (trimethylsilyl)methylamines with acidic work-up, resulting in the formation of 1,1-diarylethylenes.

We herein report a concise protocol for iterative addition of carbon nucleophiles to N,N-dialkyl carboxamides for the synthesis of α-tertiary amines.     

Reusable Co-nanoparticles for general and selective N-alkylation of amines and ammonia with alcohols

A general cobalt-catalyzed N-alkylation of amines with alcohols by borrowing hydrogen methodology to prepare different kinds of amines is reported. The optimal catalyst for this transformation is prepared by pyrolysis of a specific templated material, which is generated in situ by mixing cobalt salts, nitrogen ligands and colloidal silica, and subsequent removal of silica. Applying this novel Co-nanoparticle-based material, >100 primary, secondary, and tertiary amines including N-methylamines and selected drug molecules were conveniently prepared starting from inexpensive and easily accessible alcohols and amines or ammonia.

A general cobalt-catalyzed N-alkylation of amines with alcohols by borrowing hydrogen methodology to prepare different kinds of amines is reported.     

NickelII-catalyzed asymmetric photoenolization/Mannich reaction of (2-alkylphenyl) ketones

A diastereo- and enantioselective photoenolization/Mannich (PEM) reaction of ortho-alkyl aromatic ketones with benzosulfonimides was established by utilizing a chiral N,N′-dioxide/Ni(OTf)₂ complex as the Lewis acid catalyst. It afforded a series of benzosulfonamides and the corresponding ring-closure products, and a reversal of diastereoselectivity was observed through epimerization of the benzosulfonamide products under continuous irradiation. On the basis of the control experiments, the role of the additive LiNTf₂ in achieving high stereoselectivity was elucidated. This PEM reaction was proposed to undergo a direct nucleophilic addition mechanism rather than a hetero-Diels–Alder/ring-opening sequence. A possible transition state model with a photoenolization process was proposed to explain the origin of the high level of stereoinduction.

A diastereo- and enantioselective photoenolization/Mannich (PEM) reaction of (2-alkylphenyl) ketones with benzosulfonimides is realized by a chiral N,N′-dioxide/Ni^II complex catalyst.     

Homogeneous molecular catalysis of the electrochemical reduction of N2O to N2: redox vs. chemical catalysis

Homogeneous electrochemical catalysis of N₂O reduction to N₂ is investigated with a series of organic catalysts and rhenium and manganese bipyridyl carbonyl complexes. An activation-driving force correlation is revealed with the organic species characteristic of a redox catalysis involving an outer-sphere electron transfer from the radical anions or dianions of the reduced catalyst to N₂O. Taking into account the previously estimated reorganization energy required to form the N₂O radical anions leads to an estimation of the N₂O/N₂O˙⁻ standard potential in acetonitrile electrolyte. The direct reduction of N₂O at a glassy carbon electrode follows the same quadratic activation driving force relationship. Our analysis reveals that the catalytic effect of the mediators is due to a smaller reorganization energy of the homogeneous electron transfer than that of the heterogeneous one. The physical effect of “spreading” electrons in the electrolyte is shown to be unfavorable for the homogeneous reduction. Importantly, we show that the reduction of N₂O by low valent rhenium and manganese bipyridyl carbonyl complexes is of a chemical nature, with an initial one-electron reduction process associated with a chemical reaction more efficient than the simple outer-sphere electron transfer process. This points to an inner-sphere mechanism possibly involving partial charge transfer from the low valent metal to the binding N₂O and emphasizes the differences between chemical and redox catalytic processes.

Homogeneous electrochemical catalysis of N₂O reduction to N₂ is investigated with a series of organic catalysts and rhenium and manganese bipyridyl carbonyl complexes.     

Energy transfer (EnT) photocatalysis enabled by gold-N-heterocyclic carbene (NHC) complexes

We present the use of gold sensitizers [Au(SIPr)(Cbz)] (PhotAu 1) and [Au(IPr)(Cbz)] (PhotAu 2) as attractive alternatives to state-of-the-art iridium-based systems. These novel photocatalysts are deployed in [2 + 2] cycloadditions of diallyl ethers and N-tosylamides. The reactions proceed in short reaction times and in environmentally friendly solvents. [Au(SIPr)Cbz] and [Au(IPr)(Cbz)] have higher triplet energy (E_T) values (66.6 and 66.3 kcal mol⁻¹, respectively) compared to commonly used iridium photosensitizers. These E_T values permit the use of these gold complexes as sensitizers enabling energy transfer catalysis involving unprotected indole derivatives, a substrate class previously inaccessible with state-of-the-art Ir photocatalysts. The photosynthesis of unprotected tetracyclic spiroindolines via intramolecular [2 + 2] cycloaddition using our simple mononuclear gold sensitizer is readily achieved. Mechanistic studies support the involvement of triplet–triplet energy transfer (TTEnT) for both [2 + 2] photocycloadditions.

We present the use of gold sensitizers [Au(SIPr)(Cbz)] (PhotAu 1) and [Au(IPr)(Cbz)] (PhotAu 2) as attractive alternatives to state-of-the-art iridium-based systems.     

Catalysis enabled synthesis,structures, and reactivities of fluorinated S8-corona[n]arenes (n = 8–12)

Previously inaccessible large S₈-corona[n]arene macrocycles (n = 8–12) with alternating aryl and 1,4-C₆F₄ subunits are easily prepared on up to gram scales, without the need for chromatography (up to 45% yield, 10 different examples) through new high acceleration S_NAr substitution protocols (catalytic NR₄F in pyridine, R = H, Me, Bu). Macrocycle size and functionality are tunable by precursor and catalyst selection. Equivalent simple NR₄F catalysis allows facile late-stage S_NAr difunctionalisation of the ring C₆F₄ units with thiols (8 derivatives, typically 95+% yields) providing two-step access to highly functionalised fluoromacrocycle libraries. Macrocycle host binding supports fluoroaryl catalytic activation through contact ion pair binding of NR₄F and solvent inclusion. In the solid-state, solvent inclusion also intimately controls macrocycle conformation and fluorine–fluorine interactions leading to spontaneous self-assembly into infinite columns with honeycomb-like lattices.

NR₄F salts simultaneously catalyse and template the reaction of dithiols with C₆F₆ to generate the first fluorinated corona[n]arenes. Simple S_NAr functionalisation of the fluoraryl units easily afford diverse libraries of macrocyclic building blocks.     

Photoelectrochemical water oxidation improved by pyridine N-oxide as a mimic of tyrosine-Z in photosystem II

Artificial photosynthesis provides a way to store solar energy in chemical bonds with water oxidation as a major challenge for creating highly efficient and robust photoanodes that mimic photosystem II. We report here an easily available pyridine N-oxide (PNO) derivative as an efficient electron transfer relay between an organic light absorber and molecular water oxidation catalyst on a nanoparticle TiO₂ photoanode. Spectroscopic and kinetic studies revealed that the PNO/PNO⁺˙ couple closely mimics the redox behavior of the tyrosine/tyrosyl radical pair in PSII in improving light-driven charge separation via multi-step electron transfer. The integrated photoanode exhibited a 1 sun current density of 3 mA cm⁻² in the presence of Na₂SO₃ and a highly stable photocurrent density of >0.5 mA cm⁻² at 0.4 V vs. NHE over a period of 1 h for water oxidation at pH 7. The performance shown here is superior to those of previously reported organic dye-based photoanodes in terms of photocurrent and stability.

Stable and high photocurrent for water oxidation was achieved by an organic dye-sensitized photoanode with a pyridine N-oxide derivative as an efficient electron relay between the chromophore and molecular water oxidation catalyst.     

The effect of deuterium on the photophysical properties of DNA-stabilized silver nanoclusters

We investigated the effect of using D₂O versus H₂O as solvent on the spectroscopic properties of two NIR emissive DNA-stabilized silver nanoclusters (DNA–AgNCs). The two DNA–AgNCs were chosen because they emit in the same energy range as the third overtone of the O–H stretch. Opposite effects on the ns-lived decay were observed for the two DNA–AgNCs. Surprisingly, for one DNA–AgNC, D₂O shortened the ns decay time and enhanced the amount of µs-lived emission. We hypothesize that the observed effects originate from the differences in the hydrogen bonding strength and vibrational frequencies in the two diverse solvents. For the other DNA–AgNC, D₂O lengthened the ns decay time and made the fluorescence quantum yield approach unity at 5 °C.

We investigated the effect of using D₂O versus H₂O as solvent on the spectroscopic properties of two NIR emissive DNA-stabilized silver nanoclusters (DNA–AgNCs).     

Activation of MoS2 monolayer electrocatalysts via reduction and phase control in molten sodium for selective hydrogenation of nitrogen to ammonia

Electrochemical nitrogen fixation under ambient conditions is promising for sustainable ammonia production but is hampered by high reaction barrier and strong competition from hydrogen evolution, leading to low specificity and faradaic efficiency with existing catalysts. Here we describe the activation of MoS₂ in molten sodium that leads to simultaneous formation of a sulfur vacancy-rich heterostructured 1T/2H-MoS_x monolayer via reduction and phase transformation. The resultant catalyst exhibits intrinsic activities for electrocatalytic N₂-to-NH₃ conversion, delivering a faradaic efficiency of 20.5% and an average NH₃ rate of 93.2 μg h⁻¹ mg_cat⁻¹. The interfacial heterojunctions with sulfur vacancies function synergistically to increase electron localization for locking up nitrogen and suppressing proton recombination. The 1T phase facilitates H–OH dissociation, with S serving as H-shuttling sites and to stabilize . The subsequently couple with nearby N₂ and NH_x intermediates bound at Mo sites, thus greatly promoting the activity of the catalyst. First-principles calculations revealed that the heterojunction with sulfur vacancies effectively lowered the energy barrier in the potential-determining step for nitrogen reduction, and, in combination with operando spectroscopic analysis, validated the associative electrochemical nitrogen reduction pathway. This work provides new insights on manipulating chalcogenide vacancies and phase junctions for preparing monolayered MoS₂ with unique catalytic properties.

We describe the activation of MoS₂ in molten sodium that leads to the simultaneous formation of a sulfur vacancy-rich heterostructured 1T/2H-MoS_x monolayer electrocatalyst via reduction and phase transformation.     

One- and two-electron reduction of triarylborane-based helical donor–acceptor compounds

One-electron chemical reduction of 10-(dimesitylboryl)-N,N-di-p-tolylbenzo[c]phenanthrene-4-amine (3-B(Mes)₂-[4]helix-9-N(p-Tol)₂) 1 and 13-(dimesitylboryl)-N,N-di-p-tolyldibenzo[c,g]phenanthrene-8-amine (3-B(Mes)₂-[5]helix-12-N(p-Tol)₂) 2 gives rise to monoanions with extensive delocalization over the annulated helicene rings and the boron p_z orbital. Two-electron chemical reduction of 1 and 2 produces open-shell biradicaloid dianions with temperature-dependent population of the triplet states due to small singlet-triplet gaps. These results have been confirmed by single-crystal X-ray diffraction, EPR and UV/vis-NIR spectroscopy, and DFT calculations.

Stepwise chemical reduction of D–π–A triarylborane-based helicenes gives the corresponding monoanions and dianions with delocalized unpaired electrons. The structures were confirmed by single crystal X-ray diffraction.     

Ruthenium catalyzed β-selective alkylation of vinylpyridines with aldehydes/ketones via N2H4 mediated deoxygenative couplings

Umpolung (polarity reversal) tactics of aldehydes/ketones have greatly broadened carbonyl chemistry by enabling transformations with electrophilic reagents and deoxygenative functionalizations. Herein, we report the first ruthenium-catalyzed β-selective alkylation of vinylpyridines with both naturally abundant aromatic and aliphatic aldehyde/ketones via N₂H₄ mediated deoxygenative couplings. Compared with one-electron umpolung of carbonyls to alcohols, this two-electron umpolung strategy realized reductive deoxygenation targets, which were not only applicable to the regioselective alkylation of a broad range of 2/4-alkene substituted pyridines, but also amenable to challenging 3-vinyl and steric-embedded internal pyridines as well as their analogous heterocyclic structures.

Ruthenium-catalyzed β-selective alkylation of vinylpyridines with carbonyls (both aromatic and aliphatic ketones/aldehydes) via N₂H₄ mediated deoxygenative couplings was achieved.     

An organophotocatalytic late-stage N–CH3 oxidation of trialkylamines to N-formamides with O2 in continuous flow

We report an organophotocatalytic, N–CH₃-selective oxidation of trialkylamines in continuous flow. Based on the 9,10-dicyanoanthracene (DCA) core, a new catalyst (DCAS) was designed with solubilizing groups for flow processing. This allowed O₂ to be harnessed as a sustainable oxidant for late-stage photocatalytic N–CH₃ oxidations of complex natural products and active pharmaceutical ingredients bearing functional groups not tolerated by previous methods. The organophotocatalytic gas–liquid flow process affords cleaner reactions than in batch mode, in short residence times of 13.5 min and productivities of up to 0.65 g per day. Spectroscopic and computational mechanistic studies showed that catalyst derivatization not only enhanced solubility of the new catalyst compared to poorly-soluble DCA, but profoundly diverted the photocatalytic mechanism from singlet electron transfer (SET) reductive quenching with amines toward energy transfer (E_nT) with O₂.

An N–CH₃-selective trialkylamine oxidation to N-formamides is reported in continuous flow using gaseous O₂. A novel, enhanced-solubility dicyanoanthracene organophotocatalyst switched the photochemical mechanism from electron to energy transfer.     

Selective separation of planar and non-planar hydrocarbons using an aqueous Pd6 interlocked cage

Polycyclic aromatic hydrocarbons (PAHs) find multiple applications ranging from fabric dyes to optoelectronic materials. Hydrogenation of PAHs is often employed for their purification or derivatization. However, separation of PAHs from their hydrogenated analogues is challenging because of their similar physical properties. An example of such is the separation of 9,10-dihydroanthracene from phenanthrene/anthracene which requires fractional distillation at high temperature (∼340 °C) to obtain pure anthracene/phenanthrene in coal industry. Herein we demonstrate a new approach for this separation at room temperature using a water-soluble interlocked cage (1) as extracting agent by host–guest chemistry. The cage was obtained by self-assembly of a triimidazole donor L·HNO₃ with cis-[(tmeda)Pd(NO₃)₂] (M) [tmeda = N,N,N′,N′-tetramethylethane-1,2-diamine]. 1 has a triply interlocked structure with an inner cavity capable of selectively binding planar aromatic guests.

We report here a triply interlocked cage with the ability to encapsulate planar guests in aqueous medium. This property was then employed to efficiently separate planar and non-planar aromatic hydrocarbons by aqueous extraction.     

An electrically conductive metallocycle: densely packed molecular hexagons with π-stacked radicals

Electrical conduction among metallocycles has been unexplored because of the difficulty in creating electronic transport pathways. In this work, we present an electrocrystallization strategy for synthesizing an intrinsically electron-conductive metallocycle, [Ni₆(NDI-Hpz)₆(dma)₁₂(NO₃)₆]·5DMA·nH₂O (PMC-hexagon) (NDI-Hpz = N,N′-di(1H-pyrazol-4-yl)-1,4,5,8-naphthalenetetracarboxdiimide). The hexagonal metallocycle units are assembled into a densely packed ABCABC… sequence (like the fcc geometry) to construct one-dimensional (1D) helical π-stacked columns and 1D pore channels, which were maintained under the liberation of H₂O molecules. The NDI cores were partially reduced to form radicals as charge carriers, resulting in a room-temperature conductivity of (1.2–2.1) × 10⁻⁴ S cm⁻¹ (pressed pellet), which is superior to that of most NDI-based conductors including metal–organic frameworks and organic crystals. These findings open up the use of metallocycles as building blocks for fabricating conductive porous molecular materials.

Intrinsically electron-conductive metallocycle was synthesized. π-Radicals play a key role in constructing π-stacked columns among molecular hexagons and achieving high electrical conductivity over 10⁻⁴ S cm⁻¹ in polycrystalline pellet.